Is your feature request related to a problem? Please describe.
When selecting "All rows" in the check-all messages box and then clicking to select all messages in a large folder it can take several seconds for the messages to be visibly selected.

This can be confusing and lead to the user clicking several times without anything visibly happening.

Describe the solution you'd like
A visual cue that messages are being selected, such as a tooltip counting the number of messages while they are being checked. This would also inform the user about the number of messages that might be processed.